The Immobilizer 418 cruiser, also known as the Interdictor-class heavy cruiser, Interdictor-class medium frigate, Interdictor Dreadnaught, or simply the Interdictor, was a 600-meter-long cruiser built on the standard Vindicator-class heavy cruiser hull.

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An Immobilizer 418 shown in comparison with an Imperial-class Star Destroyer.

Interdictor cruisers, like their Vindicator-class cousins, had the appearance of small Star Destroyers. However, their lines were broken by four large globes housing gravity well projectors.[10]

The gravity wells mimicked a large mass in space and thus restricted hyperspace travel. Nearby vessels were automatically prevented from engaging their hyperdrive, and any ships passing through the area via hyperspace were forced into realspace.[3]

As well as gravity wells, the Sienar design was also equipped with 20 quad laser cannons for short range combat against other capital ships and starfighters. The Interdictor's hull was nearly identical to that of the Vindicator-class heavy cruiser and could support several additional weapons depending on mission profile. Such modifications (like those of the Corusca Rainbow, formerly the Black Asp) included the addition of ion cannons used to disable pirates and small craft attempting to flee. Enemy fire was absorbed by a deflector shield created by more than a dozen shield generators.

The Immobilizer 418 was developed as a counter to the Rebel Alliance's hyperdrive-equipped starfighters in particular, and hyperdrive-equipped vessels in general. This was primarily to counter Alliance hit and run tactics, where a small group of starfighters or light starships would emerge from hyperspace, attack preselected targets, and re-enter hyperspace before sufficient force could be brought to bear on the raiders.[3]

An Immobilizer at the site of attack, or able to access the area with a brief hyperspace jump, would be tasked to power up its gravity well projectors in an attempt to prevent the escape of the raiding party. Starfighter and capital ship support would then be used to eliminate the attackers, whose only options were to try to run for the edge of the interdiction field, or to attack the Interdictor and hope to inflict enough damage to disable the cruiser's influence. Either path would result in significant damage to the Alliance forces. Rebel pilots often mistook it for a miniature Star Destroyer when it was accompanied by Star Destroyers.[source?] It was only when they tried to escape into hyperspace that they realized it was an Interdictor. Interdictors could also be used when blockading planets or moons, such as Yavin 4.

Another duty of this class was to patrol trade routes.[5] Freighters pulled out of hyperspace by an Interdictor would be in a position to have their cargo inspected.[7] Interdictors were a welcome addition to the Imperial Customs assets, because while many smugglers and pirates wouldn't hesitate to outrun—or in some cases even outgun—an Imperial Customs Frigate, few would dare engage with a much more sizable Immobilizer 418, its gravity wells and its two squadrons of TIE starfighters. Interdictors assigned to Customs duties—such as the former Black Asp—would often have some of their quad lasers replaced by ion cannons so as to disable and capture any ship resisting inspection rather than destroy them outright.

Grand Admiral Thrawn was especially famous in connection with Interdictors for developing and refining a tactic that would be named after him—the "Thrawn Pincer" positioned the Interdictor's gravity wells along certain vectors. Other vessels—typically Star Destroyers—would be contacted, and would blindly execute a micro-hyperspace jump straight into the Interdictor's gravity wells, yanking them out of hyperspace near an enemy vessel. Much quicker than subspace movements, the Thrawn Pincer allowed reinforcements to be swiftly deployed on the battlefield and afforded a considerable element of surprise.

Although they were tremendous assets to any fleet, Interdictor cruisers suffered from some drawbacks. First and foremost, it took some time to power up and activate the gravity well projectors after the cruiser positioned itself and,[11] once activated, the tremendous simulated mass they generated made the ship very slow and cumbersome to maneuver[9]. While there was rarely enough time for an entire fleet to make the jump to hyperspace before a newly-arrived Interdictor activated its projectors, fast ships and starfighters could often escape to the edge of an active gravity well, and then make the jump to hyperspace.[11]

The gravity wells were also a large drain on the ship's energy reserves, preventing the optimal use of weaponry and shields. This flaw was exploited on several occasions. When enemy ships were able to inflict significant damage to the Interdictor, the captain was forced to choose between entrapment of the enemy and his ship's own protection. This flaw was compounded by the fact that Interdictors were poorly protected and armed for a capital ship of their size. Enemy forces could send a smaller warship, such as an Nebulon-B frigate or Thranta-class War Cruiser to weaken an Interdictor's defenses and quickly force it to power down its gravity wells to fend for itself. Although the abilities of the Immobilizer 418 were fairly effective against the missile launchers of the Marauder-class corvette often employed by the Rebel Alliance, the ship was also known for its notorious weaknesses against larger capital ships such as the Mon CalamariMC80 Star Cruiser and bombers such as the BTL Y-wing starfighter.[9]

Another problem was the fact that active gravity wells attracted space debris. An Interdictor could—and would—suffer external damage from larger pieces of such hazards. While most damage was minor—as the largest debris would be detected and fired upon by the gunnery crews—it was one reason Interdictors required more maintenance than many other fleet vehicles.[source?]

Gravity wells being used to force a ship from hyperspace.

Additionally, the engine and hyperdrive-specific areas of an Immobilizer's interior sections were crewed only by maintenance droids, and were totally unprotected. Raiders and saboteurs were thus able to infiltrate and either disassemble or destroy the vital parts of the cruiser's propulsion systems.[12]

Due to their importance and weaknesses, Immobilizer 418 cruisers were usually found on the periphery of the battle they were affecting, escorted by larger and better equipped capital ships and starfighters to protect them from harm and deliver the necessary damage.[10]

Interdictor technology was not completely limited to the Empire, however. Alliance forces made use of older CC-7700 frigates, and scientists in the Hapes Consortium reverse-engineered the gravity-well projector from captured ships; but rather than building bulky projectors, the Hapans fitted their Battle Dragons with launchers for charges known as pulse-mass mines, which produced much the same effect. However, pulse-mass mines could not be turned off or controlled like gravity wells, so this trapped the Hapans in the battle as well.

Many Vindicator cruisers were converted to Interdictors. In spite of that, only a relatively small number was used against the Rebels.[3]

An Interdictor of the Imperial fleet was essential in the capture of MoffKalast during the period of the Death Star construction. The Interdictor prevented the traitorous Moff's Star Destroyer from jumping to Hyperspace long enough for the Destroyer to be captured.[9]

An Immobilizer 418 cruiser was responsible for the elimination of the Deretta Destroyers, an Alliance to Restore the Republicstarfightersquadron, sometime before the Battle of Hoth. The squadron jumped into the target area of an Imperial depot ahead of their main Rebel strike force, where they softened up the opposition. However, the Immobilizer's presence prevented them from escaping, and the Destroyers were wiped out before the bulk of the Rebel force could arrive.[16]

The Immobilizer 418 was prohibitively expensive, reportedly costing more than an Imperial Star Destroyer[source?], and produced in very limited quantities prior to the Battle of Hoth.[1] By the time of the Battle of Endor, Sienar Fleet Systems had manufactured only a few hundred Interdictors;[source?] by comparison the Empire had approximately 25,000 Imperial Star Destroyers[18].

These vessels would not realize their full potential until several years after EmperorPalpatine's death, when they became a key component in Thrawn's attempt to defeat the New Republic. The Grand Admiral, who commanded a large number of these ships in the Outer Rim Territories, would deploy them to ambush New Republic forces. He once used an Interdictor in conjunction with his flagship, Chimaera, in an attempt to capture Luke Skywalker. However, the Jedi eluded capture, by escaping Chimaera's tractor-beam lock, racing out of range of the gravity-well projector, and making a quick jump into hyperspace.[1]

After Thrawn's defeat, New Republic technicians redesigned the Interdictor's gravity-well projectors, improving both their range and recharge rate. These new Interdictors, designated Immobilizer 418As, became one of the key components of the New Republic Defense Fleet during the Yuuzhan Vong War. During the Battle of Fondor, the New Republic supplied four Interdictors to the Hapan fleet.[1]

The Interdictor is almost always shown with its four gravity well generators represented by four complete globes, two on either flank of the hull. One early image of Thrawn's attempt to capture Luke Skywalker at Jomark, however, shows two hemispherical domes protruding along the centerline on the topsides, with two more by inference on the ventral hull.

While at odds with most subsequent representations, this rendering is as consistent with the original profile view in the Imperial Sourcebook as the more familiar four-globe variant, and another depiction of the same events at Jomark also shows the same distinct type of ship, this time seen from below—thus strengthening the claim of this type to be a canon variant rather than simply a visual inconsistency.

In the game Star Wars: Empire at War, the Immobilizer 418 is designated as a frigate, along with a wide variety of other ships smaller than Star Destroyers, and is armed with four laser cannons. The vessel is strong against Marauder-class corvettes, whose missiles it can divert, but is weak against larger capital ships and bombers. Additionally, The Official Star Wars Fact File lists the Immobilizer 418's length as 600 meters. However, this is inconsistent with the vessel's portrayal in Empire at War, in which the Immobilizer 418 is shown to be longer than the 752 meter-long Acclamator-class assault ship.